This invention relates to a polarizer system for generating circularly polarized electromagnetic waves, the polarizer system being suitable for converting a linearly polarized electromagnetic wave to a circularly polarized electromagnetic wave, and more particularly, to a polarizer system constructed of a plurality of polarizer sections arranged serially along a direction of propagation of an electromagnetic wave. Individual ones of the polarizer sections are rotated relative to each other for adjustment of differential phase between orthogonal components of an incident linearly polarized electromagnetic wave. An assembly consisting of the plurality of polarizer sections is rotated relative to the linearly polarized electromagnetic wave for adjustment of relative amplitude between the orthogonal components of the linearly polarized electromagnetic wave.
A situation of particular interest is the use of a polarizer in a satellite communication system. It is common practice in a satellite communication system to transmit data via a circularly polarized electromagnetic wave. However in practice, a precisely formed circularly polarized electromagnetic wave is hardly ever realized although the design of the polarizer is motivated to produce a circularly polarized wave. Therefore, an elliptically polarized approximation to a circularly polarized electromagnetic wave is used as a compromise so long as a specified axial ratio, namely a measure of the maximum to minimum amplitude of the elliptically polarized electromagnetic wave, is not exceeded. The specification of maximum axial ratio limits an interference between two oppositely polarized electromagnetic waves utilizing the same frequency, known in the satellite community as frequency re-use. By way of example, a linearly polarized electromagnetic wave available from the output of a rectangular waveguide is impressed into a polarizer comprising a two-fold symmetric waveguide (having square or circular cross section, by way of example) by means of a rectangular to two-fold symmetric waveguide transition. The polarizer introduces a 90 degree differential phase shift between equal amplitude orthogonal components of the impressed linearly polarized electromagnetic wave, thereby converting the linearly polarized electromagnetic wave into a circularly polarized electromagnetic wave. The polarizer operates in reciprocal fashion so that the microwave circuit can be employed both for transmission and reception of microwave signals. An ORTHOMODE transducer may be employed in place of the rectangular to two-fold symmetric waveguide whereby two orthogonal linearly polarized electromagnetic waves are available as input to the polarizer. In this case, the polarizer converts a first of the linearly polarized electromagnetic waves into one sense (right hand) of circularly polarized wave, and converts the second of the linearly polarized electromagnetic waves into the opposite sense (left hand) of polarized electromagnetic wave.
A problem arises in that some presently designed polarizers are operative based on an adjustment of the penetration of phase shifting elements into the sidewalls of the waveguide of the polarizer. This adjustment simultaneously affects both the differential phase shift and, by way of differential impedance mismatch, the relative amplitudes between the orthogonal components of a linearly polarized electromagnetic wave. For purposes of explanation, an impedance mismatch occurs when the reflections from the individual phase shifting elements do not cancel. This is disadvantageous in that, upon adjustment of the penetration of phase shifting elements so as to effectively produce a 90 degree differential phase shift between the orthogonal components of the linearly polarized electromagnetic wave, an equal amplitude ratio may no longer be present between the orthogonal components of the linearly polarized electromagnetic wave due to the impedance mismatch. For circular polarization, equality of the amplitudes of the orthogonal components and a 90 degree differential phase shift between the orthogonal components of the linearly polarized wave is required; otherwise, the electromagnetic wave is an elliptically polarized electromagnetic wave.
It has been the practice in tuning polarizers to expend a large amount of time either tuning or collecting empirical data, to overcome the foregoing disadvantages. By way of example, one adjustable phase shifting element in common use is a tuning screw penetrating the wall of the waveguide of the polarizer. This method usually requires a considerable amount of tuning time followed by post tuning soldering to avoid excess insertion loss and to prevent further movement of the tuning screws. However, soldering is not acceptable in situations where passive intermodulation (PIM) may arise. For example, a cracking in the solder may introduce such PIM. Another common method of adjusting phase shifting elements is the use of electrode discharge machining (EDM) which enables precise manufacture of intricate shapes to produce permanently installed elements such as electroformed or dip brazed pins or irises. This has the disadvantage of being nonreversible, expensive, and requiring considerable tuning time and prior collection of empirical data. In many situations where adjustable or post-fabrication tuning is not an alternative, there is often a requirement for excessively tight tolerances in order to meet performance requirements. This certainly increases the cost of manufacture.